1. Technical Field
The invention relates to ultraviolet inkjet printing. More specifically, the invention relates to the application of a visually clear, low print density after-layer of high gloss ink.
2. Description of the Prior Art
Inkjet printing involves depositing droplets of liquid ink onto a printing medium from one or more printer heads. The printer heads are coupled with a container containing ink. Ink is ejected from one or more nozzles of the print heads when a piezoelectric crystal in the print head is actuated. The piezoelectric crystal generates a pulse in the ink so that the ink expels through the nozzle as a droplet. To create the image, a carriage which holds one or more print heads scans or traverses across the printing medium, while the print heads deposit ink as the printing medium moves.
Large format printing is performed to create signs, banners, museum displays, sails, bus boards, POP applications and the like. Oftentimes consumers of large format prints prefer to choose a full or partial gloss finish to create striking displays. Gloss finishes come in various reflective intensities measured in Gloss Number. Gloss Number measures how much light is reflected at a given position. In today's art, gloss finishes are commonplace with solvent based SWF printers, but a high gloss finish is not available on today's UV printers due to the fact that the curing of the droplets of UV ink leaves a matte surface structure, rather than a very smooth finish. The relatively matte looking prints can and do suffer from a print artifact (gloss banding) which is often undesirable for many customer applications.
Gloss banding is defined as a variation in gloss between subsequent print bands on wide and super-wide format printers. This gloss variation is very visible to the eye and has a directionality component, i.e. the effect changes with viewing angle. The gloss variation is visibly most prominent when the overall gloss of the print is neither very high or very low, i.e. above gloss number value of 10 and below around 60. There have been a variety of methods employed to improve or solve gloss banding.
For example, some approaches involve use of large ink droplets to achieve a matte effect, this can also have a negative impact of print quality due to the low DPI and also in reducing color gamut, due to the less efficient use of pigment. Also formulating inks to have low drop spread and hence increased matte have been tried. This method also suffers from poor color gamut.
Another approach is to provide gloss control on an image via a curing process, in which a curable ink formulation has a variable cure by virtue of a patterned mask placed between the light source and the uncured print. The partially cured image is then fully cured via a flood lamp. The variable gloss is created due to the formation of a rough surface, caused by the variable initial cure.
According to this approach, the image usually comes out as high gloss from the printing process and the micro-patterning reduces the gloss to become more matte. Therefore, the degree of gloss is controlled by the number and size of the holes in the mask. Critical to this approach is the use of a gelling agent. The gelling agent ensures the ink is solid at temperatures below about 60° C. to reduce the absorption into paper or other absorptive substrates. Therefore, either the exposed areas become liquid during the UV mask curing or the non-exposed become liquid before they become solid in the final cure, thereby providing the pattern.
What is needed is a system of provide gloss control for images which does not use a gelling agent, or use variations in cure to obtain the level of gloss variation. Accordingly, the invention solves the problem of the “gloss banding” defect at the highest print speeds, whilst maintaining a large color gamut.